enzymes

Enzymes make molecules split up or join together
much faster than they would otherwise. To work effectively, a good
physical fit is needed between the enzyme and the other molecule(s)
– the substrate(s). Only a part of the enzyme, the active site,
often containing a coenzyme, comes into contact with other molecules.
The rest of the molecule is needed to give the enzyme the correct
shape.

The shape of enzymes can be changed by small non-substrate
molecules. A molecule of similar shape to a true substrate may compete
for the active site (A), thereby slowing down the desired reaction.
A substrate at a different site (B) changes the shape of the active
site, affecting the fir of the substrate. Some enzymes possess more
than one active site. The occupation of one or an activator may change
the shape of the second site so that it can accept the substrate (C).
An inhibitor molecule may prevent either of the sites from being used
(D).

Enzymes facilitate key processes in the biochemistry of organisms which,
otherwise, would take place too slowly for the maintenance of life. Enzymes
speed up chemical reactions of substrates
by lowering the activation energy.
They have complex tertiary structures
which are held in shape by weak chemical bonds between the polypeptide
chains.

Each enzyme has a specific surface configuration with one or more clefts
known as active sites to which only
certain substrates can bind. Each enzyme is highly specific to the reaction
it catalyzes as the substrate must fit precisely into the active site. If
the active site loses its unique shape, it can no longer provide a point
of attachment for its substrate and the enzyme is said to be denatured.
This can happen if the enzyme is subjected to temperatures or pH
levels outside of the narrow range in which it normally operates. Many enzymes
require the assistance of certain accessory substances, known as cofactors
and coenzymes, to function properly.

Shape and activity of enzymes

By changing the shapes of enzymes it is possible to inactivate them, and
thus stop certain reactions from occurring at a noticeable rate. For example,
the important protein-digesting enzyme chymotrypsin occurs in an inactive
form called chymotrypsinogen. Only when a few amino acids that make up this
protein are removed does it adopt the catalytic shape of chymotrypsin. This
change is triggered by the presence of food in the digestive tract. If the
chymotrypsin were active all the time it would rapidly digest the intestinal
wall while waiting for food to arrive.

In many biochemical processes a molecule is passed from enzyme to enzyme
before it becomes an end product. At each stage, an intermediate compound
is formed. Sometimes the final product, or one of the intermediates, can
combine with an enzyme farther back along the chain and switch it off. This
feedback is like automation in a factory that ceases production when enough
of a particular material has been made. Other small molecules may combine
with an enzyme molecule to increase its activity.

Lock-and-key
and induced models

The "lock and key" model of enzymes, first described more than a century
ago by Emil Fischer, comes surprisingly
close to the actual mechanism of enzyme-substrate interaction. In the more
recent and refined model, known as induced fit, an enzyme assumes a complementary
shape to that of its substrate only after the substrate binds to the enzyme;
hence, this is a more dynamic scenario compared to the lock-and-key hypothesis.

Summary of key enzyme properties

They are specific: because the way an enzyme works depends on its
shape, it will only work for one molecule or reaction.

They will only work within a small temperature range, e.g., enzymes
in the human body will only work around normal body temperature (37°C).

They are very sensitive to pH changes and most only work within small
pH ranges, e.g., pH6–pH7.

They can help large molecules break into smaller ones, help small
molecules join to form larger ones, or help atoms rearrange within a
molecule.